1. Field of the Invention
The present invention generally relates to transferring an image to a receiver in thermal transfer printers.
2. Description of the Prior Art
A wide variety of proposals associated with a head driving unit for use with a thermal transfer printer have heretofore been offered. One typical arrangement of a line-by-line type transfer head of a thermal transfer printer for performing two-dimensional printing, which is disclosed, e.g., U.S. Pat. No. 4,621,271 patented on Nov. 4, 1986, is provided with a shift register and a latch circuit, and the printing is effected by an enable pulse after latching transfer data for each line in the latch circuit. For the purpose of printing with gradation, for instance, the time-width of the enable pulse for driving heating resistance elements is changed according to density gradation data; or alternatively the number of pulses driving the heating element is varied.
Dye on an ink film is fused or sublimated by the heat emitted from the heating elements, thereby effecting the transfer of the dye to a receiver. In this case, however, when saturated transfer is performed in the same position for a long period of time, re-transfer phenomenon causes color dye to revert from the receiver to the ink film. Normally each line transfer is carried out while moving the receiver and the ink film.
To describe the one-line transfer in greater detail, a receiver 1 is, as depicted in FIG. 1, wound on a cylindrical roller 2 generally referred to as a drum. Roller 2 is rotated by means of a stepping motor 3, and the printing position on receiver 1 is shifted. An ink film 4 is interposed between receiver 1 and a printing head H.
During one-line printing, roller 2 shifts receiver 1 by the width W corresponding to the one-line pitch. As discussed above, receiver 1 is moved not in a continuous manner but in an intermittent manner by roller 2 moved by stepping motor 3 every time a certain number of enable pulses are generated. The number of steps with which roller 2 is moved during the one-line printing period can be varied if a pulse generator is programmed by software.
Turning next to FIG. 2, there is schematically shown the situation of how a single heating element is driven in accordance with the density gradation data on the assumption that 256 enable pulses (accordingly, the maximum density gradation is 256) are supplied during the one-line printing period, roller 2 being moved 32 steps in this period. The reference numerals ranging from 1 to 256 arranged in the uppermost part of FIG. 2 respectively indicate the timings at each of which one enable pulse is generated during the one-line printing period. Rectangular shapes laterally arranged corresponding to these generation timings indicate the intervals at which the heating elements are driven on or are not driven when the density gradations are 0, 16, 32, 48 and 256 respectively shown as D=0, D=16, D=32, D=48 and D=256. The rectangular shapes marked with oblique lines imply that the heating elements are driven, whereas the blank rectangular shapes imply that the heating elements are not driven.
In this example, 256 enable pulses are generated, and roller 2 is moved 32 steps. Therefore, it follows that the roller is shifted one step for every 8 enable pulses. For example, when the density gradation is 16, heating element 11 is driven on during the period in which the 1st to 16th enable pulses are generated. Thereafter, heating element 11 is not driven until after the 256th enable pulse is generated. Meanwhile, roller 2 moves step by step at the time when the 8th, 16th, . . . , 256th enable pulses are generated.
After roller 2 has shifted 32 steps, the heating element is located on the next printing line. If a transfer image extends over two lines or more, the operations discussed above are repeated.
FIG. 3 shows a relation of a transfer print between the density gradation and the optical density according to a conventional thermal printer. It can be observed from FIG. 3 that if the density gradation increases to some extent, the saturation takes place when the optical density comes to around 1.4. Consequently, the maximum density of the image is low and the resulting image exhibits poor contrast. It can also be considered that the voltage impressed on the head can be increased to raise the upper limit of the optical density. If the voltage is augmented, the re-transfer phenomenon appears, and it would be impossible to raise the upper limit of the optical density further.
In the conventional image transfer apparatus, as can be understood from the description given above in conjunction with FIG. 2, the driving of the heating elements begins with the first enable pulse. The period in which each heating element is driven is defined by: EQU T.times.(d/D)
where T is the printing period for one line, d is a desired density gradation and D is the maximum density gradation. If the desired density gradation d is, for example, substantially one-half of the maximum density gradation D, the printing is effected only on the first half of the printing space for one line. The second half of the printing space is not printed. Hence, dye is not uniformly transferred across the entire line width W.